To me, it just felt like the pacing was off, and that a lot of stuff got left out, or was left unresolved. In the first half, they introduce a bunch of side characters that you think will play some important role in the story, then they just throw most of them out and head to Easter Island, where they introduce a new group of side characters, and never do much with them either. It's like they planned out the anime for a couple seasons, but could only get funding for 12 episodes, so they just cut out all the side stories and edited together what they had left.

And yeah, animation quality seemed to take a dive as the series went on. I wouldn't say the series was bad, but it didn't live up to the potential seen in the early episodes. I might check out the manga, since it likely includes a lot of content cut from the anime, and probably gives the side characters more of a reason to be there.